1. Field of the Invention
The invention relates in general to a scanner and exposure control method thereof, and more particularly to a scanner, of which the driving source has speed non-uniformity issue, and exposure control method thereof.
2. Description of the Related Art
Referring to FIG. 1A, a schematic structure diagram of a conventional scanner is shown. The scanner 100 includes a scan flatbed 110, an optical module 120, a driving source 130, and an application specific integrated circuit (ASIC) 140. The driving source 130 drives the optical module to move at a constant speed, in the meanwhile the photo sensing device 122, such as a charge coupled device (CCD), captures images from several scan lines of the to-be-scanned document 111 on the scan flatbed 110. The driving source 130 has an encoder 132 for outputting a position signal PF. The ASIC 140 controls the driving source according to the position signal PF to exactly position the moving optical module 120, and thus images captured from the scan lines can be assured of having a uniform width.
However, due to slight variation on the constant speed control of the driving source, the exposure time for each of the scan lines can be varied. As shown in FIG. 1B, the scanner 100 determines the exposure time T1, T2, and T3 for the photo sensing device 122 reading the to-be-scanned document 111 according to the position signal PF. If the driving source drives the optical module 120 to move at a predetermined constant speed, the exposure time for a scan line is set to be T1=t2−t0. If the driving source 130 drives the optical module 120 to read a certain scan line at a speed larger than the predetermined constant speed, the exposure time for this scan line is set to be T2=t1−t0. If the driving source drives the optical module to read a certain scan line at a speed smaller than the predetermined constant speed, the exposure time for this scan line is set to be T3=t3−t0. Since the starting point and ending point of the exposure for the scan lines are both controlled by the feedback position signals, the exposure time T1, T2, and T3 for different scan lines will be unequal. According to the formula: exposure amount=light density of the to-be-scanned document 111×responsibility of the photo sensing device 122×exposure time×gain of the analog front end (AFE), the exposure amount for each scan line is not constant, thereby reducing scan image quality.
Referring to FIG. 1C, a circuit block diagram of the scanner using a DC motor for the position feedback control disclosed by U.S. Pat. No. 6,037,584, is shown. For the light density of the to-be-scanned document 111 and the responsibility of the photo sensing device is usually constant in the whole scan process, in order to solve the above-mentioned issue of unequal exposure amount for each scan line, the invention disclosed in the cited patent dynamically adjusts the AFE gain to compensate the different exposure time such that the exposure amount for each scan line is equal.
The optical module 170 of the scanner 150 is driven to scan the to-be-scanned document (not shown in the figure) by the DC motor 160. The DC motor 160 has an encoder 162 for outputting a position signal PF to the timer 182 of the ASIC 180. The exposure control unit 184 controls the exposure time of the CCD 172 in the optical module 170 for reading each scan line of the to-be-scanned document according to the position signal PF. The data read by the CCD 172 are output to the analog amplifying unit for further amplification. The ASIC 180 of the cited patent further includes a gain control unit 186. The gain control unit 186 dynamically adjusts the AFE gain of the analog amplifying unit 174 to compensate the varied exposure amount due to the unequal exposure time for each scan line according to the control of the exposure control unit 184 and the timer 182.
However, the scanner disclosed by the above-mentioned patent requires extra complicated circuits, such as the gain control unit 186, thereby increasing the manufacturing cost, in order to dynamically adjust the AFE gain. Moreover, the AFE gain compensation disclosed in the patent is a kind of analog signal compensation requiring extra time for converting digital signals to analog signals. For the analog signals are easily interfered by the exterior noise in the transmission process, and the linearity of the analog signal compensation is poor, the scan image quality cannot be effectively improved.